=Crocodiles and alligators (Crocodilia).=--The crocodiles and alligators are reptiles familiar by name and appearance, though seen in nature only by inhabitants or visitors in tropical and semitropical lands. In the United States there are two species of these great reptiles, the American crocodile (_Crocodilus americanus_), living in the West Indies and South America and occasionally found in Florida, and the American alligator (_Alligator mississippiensis_), common in the morasses and stagnant pools of the southern States. The alligator differs from the crocodiles in having a broader snout. It is rarely more than twelve feet long. The best-known crocodile is the Nile crocodile, which is not limited to the Nile, but is found throughout Africa. In the Ganges of India is found another member of this group of reptiles called the gavial. It is among the largest of the order, reaching a length of twenty feet. The crocodiles, alligators, and gavials comprise not more than a score of species altogether, but because of their wide distribution, great size, and carnivorous habits they are among the most conspicuous of the larger living animals. They live mostly in the water, going on land to sun themselves or to lay their eggs. They move very quickly and swiftly in water but are awkward on land. Fish, aquatic mammals and other animals which occasionally visit the water are their prey. The gavial and Nile crocodile are both known to attack and devour human beings, and these species annually cause a considerable loss of life. But few such fatalities, however, are accredited to the American alligator. FOOTNOTE: [17] By many zoologists the lizards and snakes are held to form two distinct orders, Lacertilia and Ophidia. CHAPTER XXVII BRANCH CHORDATA (_Continued_). CLASS AVES: THE BIRDS THE ENGLISH SPARROW (_Passer domesticus_) TECHNICAL NOTE.--The English sparrow may be found now in cities and villages all over the United States. It has become a veritable pest, and the killing of the few needed for the laboratory may be looked on as desirable rather than deplorable, as is the killing of birds in almost all other cases. The males have a black throat, with the other head-markings strong and contrasting (black, brown, and white), while the females have a uniform grayish and brownish coloration on the head. Specimens are best taken alive, as shooting usually injures them for dissection. One can rely on the ingenuity of the boys of the class to procure a sufficient number of specimens. Observations on the habits of the birds should be made by the pupils as they go to and from school. For dissection use fresh specimens if possible. If desirable a pigeon or dove may be used in place of the sparrow.